1. Field of the Invention
The present invention relates to a method and system for oxidizing surfaces and, in particular, is directed to oxidizing silicon-containing and other semiconductor wafers in a high-density plasma chemical vapor deposition tool.
2. Description of Related Art
In general, an oxidation process is performed by heating wafers in an ambient of oxygen-bearing gas. Typical oxygen-bearing gases are molecular oxygen O2, water vapors H2O, nitrous oxide N2O, nitric oxide NO, and ozone O3. Oxidation reactors can be roughly divided into single wafer tools and furnaces. A typical oxidation furnace processes many wafers at once. Modern oxidation furnaces can process 100-200 200-mm-diameter wafers in one batch. Due to the large batch size, the furnaces can support relatively slow processes without affecting throughput. A typical furnace throughput is 50-100 wafers per hour. On the other hand, single wafer tools process one wafer at a time. Single wafer tools allow for a greater manufacturing flexibility, for they can support a different process per given wafer. A manufacturing facility that processes a large mix of different products can greatly benefit from the flexibility of single wafer oxidation tools. In order to support a competitive throughput as compared to that of a furnace, single wafer tools employ fast oxidation processes. A typical throughput of a single wafer oxidation tool is 20-40 wafers per hour. Single wafer oxidation processes longer than 5 minutes are highly undesirable. In fact, a preferred single wafer process is less than 2 minute long.
General trends in micro- and nanofabrication are directed to the reduction of thermal budget, processing of large substrates, and increased three-dimensional integration. Reduced thermal budget allows for sharp dopant profiles and prevents chemical reaction and intermixing between adjacent dissimilar materials. Consequently, the device features can be made smaller without loss of structural integrity. Larger substrates allow for more devices to be manufactured per given processing sequence. As the result, the manufacturing cost per given device is reduced. Integration of various devices into the third dimension (perpendicular to the wafer surface) offers even greater device density.
One way to speed up the oxidation process is to use a highly reactive oxygen-bearing gas that can rapidly react with the substrate at a low temperature. The most reactive oxygen-bearing gas is free oxygen radical or atomic oxygen. In order to produce a substantial amount of atomic oxygen, one has to dissociate more stable oxygen-bearing gas (e.g. O2, O3, NO, N2O) with the aid of some excitation. The excitation can be in the form of electrical discharge, flux of photons (photo assisted), electron beam, or localized intense heat. The atomic oxygen is inherently unstable substance and may quickly recombine without reaching the substrate. Furthermore, providing a uniform distribution of atomic oxygen over relatively large substrate (200-mm in diameter and larger) is a challenge. Accordingly, the atomic oxygen has to be first produced with the aid of some excitation, than delivered to the substrate with minimal recombination losses, and finally redistributed over the substrate surface in a manner that ensures an acceptable uniformity of the process.
There are several known processes and tools that employ atomic oxygen for a fast, low temperature oxidation process.
A low temperature, charge-free process for forming oxide layers was disclosed in U.S. Pat. No. 4,474,829, which utilized an oxygen-containing precursor and exposed it to radiation of a selected wavelength to cause direct dissociation to generate oxygen solely in atomic form. However, this process is relatively complex and requires specially-built tools, not readily commercially available tools such as an RTO oxidation tool.
Gronet et al. U.S. Pat. No. 6,037,273 discloses an apparatus to carry out an in-situ steam generation (ISSG) oxidation technique. Gronet discloses that the in-situ steam generation rapid thermal processor (a single wafer tool) is well suited for high volume semiconductor manufacturing due to a superior temperature uniformity, fast temperature ramps, high throughput, and acceptable safety record. Gronet discloses that a substrate can be placed in such a reactor and then oxidized using the in-situ generated steam. Gronet discloses a fast oxidation of a substrate having a Si layer. Tews et al. U.S. Pat. No. 6,358,867 teaches that the oxidation process conducted in in-situ steam generation rapid thermal processor shows little orientation dependence. Tews et al. teaches that the absence of orientation dependence is the earmark of atomic oxidation. Tews et al. refers to the ISSG oxidation technique as free radical enhanced rapid thermal oxidation (FRE RTO). In a related application Ser. No. 09/874,144, Ballantine et al. teaches that the ISSG process is capable of rapidly oxidizing very stable silicon nitride. Ballantine et al. teaches that the rapid oxidation of SiN can be only performed with the aid of some excitation.
Oxidation by use of plasmas has also been disclosed in U.S. Pat. Nos. 4,232,057, 4,323,589, 5,330,935, 5,443,863, 5,872,052, 5,913,149, 5,923,948 and 6,165,834. While the disclosed plasma oxidation methods have shown some promise, they have resulted in relatively low oxide growth-rate, have been limited to relatively small substrate surface areas or low density plasma (which gives poor throughput for thicker films) or have had other problems which have made them unable to compete with the commercially available RTO oxidation tools.
FRE RTO or ISSG process can provide substantial amount of atomic oxygen over 200-mm substrate resulting in a number of useful atomic oxidation processes such as orientation independent oxidation of silicon and fast oxidation of silicon nitride. Nevertheless, the process is limited the high substrate temperature (above 600C) because the atomic oxygen is generated within the chamber as a secondary byproduct of multi-step reaction between hydrogen H2 and oxygen O2. Furthermore, safety requirements limit the process substrate temperature to even higher temperature (above 800C). This temperature range is relatively high compare to the atomic oxidation processes where the radicals are created in a plasma discharge.